Field of the Invention
Embodiments of the present invention generally relate to methods to detect endpoints for both a photoresist layer and an absorber layer in an etching process for the fabrication of photomasks useful in the manufacture of integrated circuits.
Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
The next generation photomask as further discussed below is formed on a low thermal expansion glass or a quartz substrate having a multilayer film stack disposed thereon. The multilayer film stack may include at least an absorber layer and a photomask shift mask layer. When manufacturing the photomask, a photoresist layer is disposed on the film stack to facilitate transferring features into the film stack during the subsequent patterning processes. During the patterning process, the circuit design is written onto the photomask by exposing portions of the photoresist to extreme ultraviolet light or ultraviolet light, making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed, allowing the underlying film stack exposed through the remaining photoresist to be etched. The etch process removes the film stack from the photomask at locations where the resist was removed, i.e., the exposed film stack is removed.
During processing, endpoint data from the etching of the film stack for manufacturing photomasks may be used to determine whether the process is operating according to required specifications, and whether the desired results such as etch uniformity and feature critical dimensions are achieved. Since each photomask generally has its own set of features or patterns, different film stacks on the photomask being etched may yield different endpoint data upon different materials being used in the film stack, thereby making it difficult to determine if the desired etch results are obtained for a specific photomask manufacture process. Furthermore, during an etching process, the etching rate for etching the photoresist layer and the film stack for the photomask may be different. Accordingly, when directing a radiation to the photoresist layer and the film stack on the photomask, different thickness variation between the photoresist layer and the film stack may generate different reflective or transmissive signal to the endpoint data, therefore, making it even more difficult to determine an accurate endpoint for both the photoresist layer and the photomask etching process without interfered by the photoresist thickness variation. Thus, an accurate etching process endpoint control to the film stack disposed on the photomask and the photoresist layer thickness remaining on the photomask after the etching process for advanced PSM (phase-shift mask) or EUV technology is highly desirable.
Therefore, there is an ongoing need for improved etching endpoint process control in photomask fabrication, including improved apparatus and methods for collecting etch rate data and determining process endpoints.